Metal components, such as compressor blade dovetails of gas turbines, are prone to failure by both fatigue and fretting fatigue. Fatigue can be described as the process by which cyclic loads below a material's tensile strength initiate and propagate surface cracks. Fretting fatigue can be described as a specific form of fatigue in which small amplitude sliding motion, for example, between about 50 and about 200 micrometers, initiates and propagates cracks. The sliding motion can exacerbate the usual fatigue process through production of abrasive, oxidized wear product. Fretting fatigue can occur in components such as aerofoil dovetails and/or the attachment points in gas or steam turbine rotors. Both ordinary and fretting fatigue cracks are surface-initiated microcracks that propagate to the interior of the component. When components have high temperature gradients with the surface being at a greater temperature than the interior portions, fretting fatigue cracks are more likely to be formed. Compressive residual stress proximal to the surface of the components reduces the likelihood that both fatigue and fretting fatigue cracks will form.
In a known system, laser shock processing can be used to improve the fretting fatigue resistance by increasing compressive residual stress at the surface of a component. Using laser shock processing suffers from several drawbacks. For example, laser shock processing can damage components such as blade dovetails due to high intensity shock waves generated. The shock waves reflect on surfaces of the base material generating a tensile stress. The tensile stress can propagate existing flaws and cracks leading to failure of the base material. In addition, laser shock processing adds undesirable costs.
In another known system, a coating is applied to a compressor blade dovetail to improve the fretting fatigue resistance. Application of the coating suffers from several drawbacks. For example, application of the coating Alumazite can have a premature failure due to the tensile nature of stress at an interface between the coating and a base material (for example, a martensitic stainless steel such as an alloy including about 15.5% chromium, about 6.3% nickel, about 0.8% molybdenum, about 0.03% carbon, and a balance of iron). This can result in propagation of cracks into the base material when the coating spalls off cracks. This can reduce the corrosion resistance of the base material and permit crevice corrosion of the base material. In addition, the coating can result in additional undesirable costs.
In another known system, water jet impact is used to treat a component. The water jet impact removes debris and cleans the surface of the component. Water jet impact suffers from several drawbacks. For example, water jet impact does not improve fretting fatigue resistance and water jet impact does not increase the compressive residual stress near the surface of the component.
A system and method capable of improving the fretting fatigue resistance of a component that does not suffer from the above drawbacks would be desirable in the art.